1. Field of the Invention
The present invention relates to a drive arrangement for an absorber rod of a nuclear reactor. More particularly, the invention concerns an absorber unit comprising an absorber rod, an absorber rod drive and at least one drive cylinder accessible to a fluid drive medium. The absorber rod is arranged in the pressure vessel of the nuclear reactor and is preferably cooled by the reactor cooling system.
2. Background of the Prior Art
In the known drives of this type serving for the insertion and extraction of absorber rods into the core area of a nuclear reactor, the drive piston must be exposed constantly to the driving medium in order to maintain the absorber rod against the over-pressure prevailing in the pressure vessel in the completely inserted or partially inserted state. This presents undesirable difficulties for control of the absorber rod, for example, in the event of a drop in the pressure of the driving medium (occurring as the result of the leakage of supply lines or as the result of changes in the internal pressure prevailing in the pressure vessel) the absorber rod will be driven into a retracted position and quite possibly out of the pressure vessel itself.
It is, therefore, an object of the invention to provide a drive arrangement of the above-mentioned type in a simple and cost-effective manner whereby the absorber rod is maintained in its position in case of a drop in or failure of the pressure of the driving medium or during variations of the internal pressure of the pressure vessel.
This object is attained according to the invention with a drive of the above-mentioned type also including an absorber unit comprising an absorber rod, an absorber rod drive and at least one drive cylinder. The absorber unit is equipped with at least one equalizing surface extending transversely to the longitudinal axis of the absorber rod. The equalizing surface faces the direction opposite the first surface penetrating the reactor and is constructed of material capable of withstanding exposure to the medium contained in the pressure vessel and/or the cooling medium of the absorber rod. The size and arrangement of the equalizing surface is chosen so that the equalizing force applied to the equalizing surface by the medium or cooling medium, respectively, at least substantially compensates the force acting from the medium of the pressure vessel on the absorber rod in the outward, axial direction.
The arrangement of such an equalizing surface accessible to the medium of the pressure vessel or the cooling medium of the absorber rod applies an equalizing force to the absorber unit. That is, the equalizing force effectively neutralizes the opposing force exerted by the medium of the pressure vessel on the absorber rod which acts to drive the rod from the pressure vessel. The size of the equalizing surface is chosen so that the equalizing force may be adapted to the force pressing against the absorber rod. This assures the maintenance of the absorber rod in its existing position in the case of a drop of pressure of the driving medium or in the case of fluctuations of the pressure inside the pressure vessel.
In certain embodiments of the present invention, it is advantageous to generate the equalizing pressure by means of the medium contained in the pressure vessel. It is advantageous in this instance to arrange the equalizing surface on an upper portion of the absorber unit extending axially to the absorber rod. The extension penetrates the cylindrical area surrounding it and terminates in a space contiguous with the inner area of the vessel.
Another, equally advantageous embodiment of the invention consists of arranging the equalizing surface on the outermost end of the drive piston utilized for insertion and extraction of the absorber rod. Again, the cylinder area surrounding the equalizing surface is contiguous with the inner area of the pressure vessel.
In an embodiment employing the use of a hollow, cylindrical absorber rod, the internal space thereof is preferably exposed to a cooling medium and has at least one radial opening for the exit of the cooling medium in its bottom area. It is particularly advantageous to direct the cooling medium into the hollow cylindrical absorber rod making the inside bottom surface of the hollow cylinder the equalizing surface.
In order to equalize or neutralize the forces acting on the absorber unit of this embodiment, it is advisable to render the sum of the equalizing forces and the sum of the surfaces exposed to the axially outward pressure of the medium of the pressure vessel, possibly with the consideration of the weight of the absorber unit, approximately equal.
In this manner, both the dynamic pressure of the flowing cooling medium and the pressure of the medium in the pressure vessel itself act upon the bottom of the hollow cylindrical absorber rod, thus producing the equalizing force desired. Because the flow velocity of the cooling medium and thus the dynamic pressure applied cannot be freely selected, the weight of the absorber unit may be additionally employed in the equalization of the driving force.
Further advantages and characteristics of the invention will be found in the description of examples following hereinbelow and in connection with the schematic drawings.